Lazy tournaments and multidegrees of a projective embedding of $\overline{M}_{0,n}
We provide a new geometric interpretation of the multidegrees of the (iterated) Kapranov embedding $\Phi_n:\overline{M}_{0,n+3}\hookrightarrow \mathbb{P}^1\times \mathbb{P}^2\times \cdots \times \mathbb{P}^n$, where $\overline{M}_{0,n+3}$ is the moduli space of stable genus $0$ curves with $n+3$ mar...
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Zusammenfassung: | We provide a new geometric interpretation of the multidegrees of the
(iterated) Kapranov embedding $\Phi_n:\overline{M}_{0,n+3}\hookrightarrow
\mathbb{P}^1\times \mathbb{P}^2\times \cdots \times \mathbb{P}^n$, where
$\overline{M}_{0,n+3}$ is the moduli space of stable genus $0$ curves with
$n+3$ marked points. We enumerate the multidegrees by disjoint sets of boundary
points of $\overline{M}_{0,n+3}$ via a combinatorial algorithm on trivalent
trees that we call a lazy tournament. These sets are compatible with the
forgetting maps used to derive the recursion for the multidegrees proven in
2020 by Gillespie, Cavalieri, and Monin.
The lazy tournament points are easily seen to total $(2n-1)!!=(2n-1)\cdot
(2n-3) \cdots 5 \cdot 3 \cdot 1$, giving a natural proof of the fact that the
total degree of $\Phi_n$ is the odd double factorial. This fact was first
proven using an insertion algorithm on certain parking functions, and we
additionally give a bijection to those parking functions. |
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DOI: | 10.48550/arxiv.2108.00050 |